I downloaded the Kindle for PC fix today and it refused to accept my (valid) username and password. After a bit of debugging I found one of the SSL certificates was missing on my fairly new Windows7 PC. Here's how I fixed it.

1. In IE, go to https://firs-ta-g7g.amazon.com. If you are affected by the same issue as me you will get a certificate error screen. Click the 'go to the website' link.